This can be variable depending on your exam board, and what your teacher wants from you, but there are some things that never change.
Structure and Plan
It's best to begin the essay with a structure in mind. It doesn't have to be complicated at all; 'Introduction', 'Reasons for', 'Reasons against', 'Conclusion' is fine. As for a plan, jot down a bullet point about each section, a sort of summary of what each paragraph should tell the reader.
You should take this time to read the question carefully, noting any dates you need to take into account. Reread sources if they're given to you, and make sure you refer to the source in your answer.
Writing down your ideas
It's easier to take a firm stance on the question than to be on the fence. Even if you don't fully agree, as long as you can write an effective argument about it, it doesn't matter.
What an essay is all about
You have an opinion of the question, and you want to convince the reader about your point of you. However, the best way to explain anything to anyone is to reason with evidence. The evidence in history is the knowledge bit, while the argument is the essay skills. Keep this in mind as you write and look over your answer.
Remember, you are not telling a story or reciting facts; you are making an argument, and taking bits and pieces of information to come up with a cohesive explanation for your opinion.
Keep it simple
Always prefer a simple, straightforward answer to an over-flourished one (this includes waffling). For example:
a) The Hitler Youth was a method the government used to instil propoganda values into children. It was also a way of controlling their behaviour.
b) The Hitler Youth took many camping trips and earned badges for their work, and many children joined it. Sometimes, when they came home from meetings, they surprised their parents with their new way of thinking. This style of thinking is what the government wanted to them to have.
Answer a) gets to the point and will save you time.
Links
Making long and short term causation links will show your grasp of knowledge and your ability to think about levels of causation.
Finally - confidence!
There are no right or wrong answers in history - just well argued and poorly aruged ones! Have confidence in your answer, and it will show through! An easy way to attain this confidence in writing? Practice a lot (make sure to time yourself, including time to plan!) and learn your stuff!